It is well recognized by persons skilled in the art of oil recovery that there are formations which contain petroleum whose viscosity is so great that little or no primary production is possible. Some form of supplemental oil recovery must be applied to these formations which decreases the viscosity of the petroleum sufficiently that it will flow or can be displaced through the formation to production wells and then through to the surface of the earth. Thermal recovery techniques are quite effective viscous oil recovery methods, and steam flooding is the most successful thermal oil recovery technique yet employed in commercial application. Steam may be utilized for thermal stimulation for viscous oil formations by means of a "huff and puff" technique in which steam is injected into a well, allowed to remain in the formation for a soak period, and then oil is recovered from the formation by means of the same well as was used for steam injection. Another technique employing steam stimulation is a steam drive or steam throughput process, in which steam is injected into the formation on a more or less continuous basis by means of an injection well and oil is recovered from the formation from a spaced-apart production well. This technique is somewhat more effective in many applications than the "huff and puff" steam stimulation process since it both reduces the viscosity of the petroleum and displaces petroleum through the formation, thus stimulating oil production from a production well. While this process is very effective with respect to the portions of the recovery zone between the injection well and production well through which the steam travels, poor vertical conformance is often experienced in steam drive oil recovery processes. A major cause of poor vertical conformance is associated with the fact that steam, being of lower density than other fluids present in the permeable formation, migrates to the upper portion of the permeable formation and channels across the top of the oil formation to the remotely located production well. Once steam channeling has occurred in the upper portion of the formation, the permeability of the steam-swept zone is increased due to the desaturation or removal of petroleum from the portions of the formation through which steam has channeled. Thus subsequently-injected steam will migrate almost exclusively through the steam-swept channel and very little of the injected steam moves into the lower, unswept portions of the formation, and thus very little additional petroleum is recovered from the lower portions of the formation. While steam drive processes effectively reduce the oil saturation in the portion of the formation through which they travel by a significant amount, a portion of the recovery zone between the injection and production systems actually contacted by steam is often less than 50 percent of the total volume of that recovery zone, and so a significant amount of oil remains in the formation after completion of the steam drive oil recovery process. The severity of the poor vertical conformance problem increases with the thickness of the oil formation and with the viscosity of the petroleum contained in the oil formation.
In view of the foregoing discussion, and the large deposits of viscous petroleum from which only a small portion can be recovered because of the poor conformance problem, it can be appreciated that there is a serious need for a steam drive thermal oil recovery method suitable for use in recovering viscous petroleum from relatively thick formations with improved vertical conformance.